1. Field of the Invention
The present invention relates generally to methods for roll-to-roll deposition of optically transparent and high conductivity metallic films for use in plastic display electrodes and other analogous devices, and to composite films made using those methods.
2. General Background and State of the Art
A liquid crystal display (LCD) is a type of flat panel display used in various electronic devices. Generally, LCDs comprise two sheets of polarizing material with a liquid crystal solution therebetween. Each sheet of polarizing material typically comprises a substrate of glass or transparent plastic; a liquid crystal (LC) is used as optical switches. The substrates are usually manufactured with transparent electrodes, typically made of indium tin oxide (ITO) or another conductive metallic layer, in which electrical xe2x80x9cdrivingxe2x80x9d signals are coupled. The driving signals induce an electric field which can cause a phase change or state change in the LC material, the LC exhibiting different light-reflecting characteristics according to its phase and/or state.
Liquid crystals can be nematic, smectic, or cholesteric depending upon the arrangement of the molecules. A twisted nematic cell is made up of two bounding plates (usually glass slides or plastic plates), each with a transparent conductive coating (such as ITO or another conductor) that acts as an electrode, spacers to control the cell gap, two cross polarizers (the polarizer and the analyzer), and nematic liquid crystal material. Twisted nematic displays rotate the direction of the liquid crystal by 90xc2x0. Super-twisted nematic displays employ up to a 270xc2x0 rotation. This extra rotation gives the crystal a much deeper voltage-brightest response, also widens the angle at which the display can be viewed before losing much contrast. Cholesteric liquid crystal (CLC) displays are normally reflective (meaning no backlight is needed) and can function without the use of polarizing films or a color filter. xe2x80x9cCholestericxe2x80x9d means the type of liquid crystal having finer pitch than that of twisted nematic and super-twisted nematic. Sometimes it is called xe2x80x9cchiral nematicxe2x80x9d because cholesteric liquid crystal is normally obtained by adding chiral agents to host nematic liquid crystals. Cholesteric liquid crystals may be used to provide bistable and multistable displays that, due to their non-volatile xe2x80x9cmemoryxe2x80x9d characteristic, do not require a continuous driving circuit to maintain a display image, thereby significantly reducing power consumption. Feroelectric liquid crystals (FLCs) use liquid crystal substances that have chiral molecules in a smectic C type of arrangement because the spiral nature of these molecules allows the microsecond switching response time that makes FLCs particularly suited to advance displays. Surface-stabilized feroelectric liquid crystals (SSFLCs) apply controlled pressure through the use of a glass plate, suppressing the spiral of the molecules to make the switching even more rapid.
Some known LCD devices include chemically-edged transparent, conductive layers overlying a glass substrate. See, e.g., U.S. Pat. No. 5,667,853 to Fukuyoshi et al., incorporated herein by reference. Unfortunately, chemical etching processes are often difficult to control, especially for plastic films. Such processes are also not well-suited for production of the films in a continuous, roll-to-roll manner, on plastic substrates.
There are alternative display technologies to LCDs that can be used, for example, in flat panel displays. A notable example is organic or polymer light-emitting devices (OLEDs) or (PLEDs), which are comprised of several layers in which one of the layers is comprised of an organic material that can be made to electroluminesce by applying a voltage across the device. An OLED device is typically a laminate formed in a substrate such as glass or a plastic polymer. A light-emitting layer of a luminescent organic solid, as well as adjacent semiconductor layers, are sandwiched between an anode and a cathode. The semiconductor layers can be whole-injecting and electron-injecting layers. PLEDs can be considered a subspecies of OLEDs in which the luminescent organic material is a polymer. The light-emitting layers may be selected from any of a multitude of light-emitting organic solids, e.g., polymers that are suitably fluorescent or chemiluminescent organic compounds. Such compounds and polymers include metal ion salts of 8-hydroxyquinolate, trivalent metal quinolate complexes, trivalent metal bridged quinolate complexes, Schiff-based divalent metal complexes, tin (IV) metal complexes, metal acetylacetonate complexes, metal bidenate ligand complexes incorporating organic ligands, such as 2-picolylketones, 2-quinaldylketones, or 2-(o-phenoxy) pyridine ketones, bisphosphonates, divalent metal maleonitriledithiolate complexes, molecular charge transfer complexes, rare earth mixed chelates, (5-hydroxy) quinoxaline metal complexes, aluminum tris-quinolates, and polymers such as poly(p-phenylenevinylene), poly(dialkoxyphenylenevinylene), poly(thiophene), poly(fluorene), poly(phenylene), poly(phenylacetylene), poly(aniline), poly(3-alkylthiophene), poly(3-octylthiophene), and poly(N-vinylcarbazole). When a potential difference is applied across the cathode and anode, electrons from the electron-injecting layer and holes from the hole-injecting layer are injected into the light-emitting layer; they recombine, emitting light. OLEDs and PLEDs are described in the following United States patents, all of which are incorporated herein by this reference: U.S. Pat. No. 5,707,745 to Forrest et al., U.S. Pat. No. 5,721,160 to Forrest et al., U.S. Pat. No. 5,757,026 to Forrest et al., U.S. Pat. No. 5,834,893 to Bulovic et al., U.S. Pat. No. 5,861,219 to Thompson et al., U.S. Pat. No. 5,904,916 to Tang et al., U.S. Pat. No. 5,986,401 to Thompson et al., U.S. Pat. No. 5,998,803 to Forrest et al., U.S. Pat. No. 6,013,538 to Burrows et al., U.S. Pat. No. 6,046,543 to Bulovic et al., U.S. Pat. No. 6,048,573 to Tang et al., U.S. Pat. No. 6,048,630 to Burrows et al., U.S. Pat. No. 6,066,357 to Tang et al., U.S. Pat. No. 6,125,226 to Forrest et al., U.S. Pat. No. 6,137,223 to Hung et al., U.S. Pat. No. 6,242,115 to Thompson et al., and U.S. Pat. No. 6,274,980 to Burrows et al.
In a typical matrix-address light-emitting display device, numerous light-emitting devices are formed on a single substrate and arranged in groups in a regular grid pattern. Activation may be by rows and columns, or in an active matrix with individual cathode and anode paths. OLEDs are often manufactured by first depositing a transparent electrode on the substrate, and patterning the same into electrode portions. The organic layer(s) is then deposited over the transparent electrode. A metallic electrode can be formed over the electrode layers. For example, in U.S. Pat. No. 5,703,436 to Forrest et al., incorporated herein by reference, transparent indium tin oxide (ITO) is used as the whole-injecting electrode, and a Mgxe2x80x94Agxe2x80x94ITO electrode layer is used for electron injection.
Previous methods of manufacturing such films have not succeeded in manufacturing such films by a continuous process on flexible substrates, yielding films with desirable properties such as high transmittance, low electrical resistance, and stability to temperature and humidity.
For example, PCT Publication No. WO 99/36261, by Choi et al. (Polaroid Corp.) published on Jul. 22, 1999, and incorporated by this reference, describes the deposition of ITO/Au/Ag/Au/ITO multilayered films on polymer (Arton substrate). In this multilayered structure, the Ag layer has a thickness of 10-15 nm and the two ITO layers have a thickness of 35-50 nm. As compared with ITO/Ag/ITO multilayered films, an Au/Ag/Au sandwiched layer works as a conductive layer in the multilayered structure and exhibits an enhanced corrosion resistance as the 1-1.5 nm Au layers prevent the water or oxygen from entering the Au/Ag interfacial area. It was reported that the ITO/Au/Ag/Au/ITO films have a sheet resistance below 10 xcexa9/square and a transmittance above 80%. However, the deposition process for these multilayered films is much more complicated than the deposition process for ITO/Ag/ITO films.
U.S. Pat. No. 5,667,853 to Fukuyoshi et al., incorporated herein by reference, describes the formation of InCeO/Ag/InCeO films in which the InCeO layers have a thickness of about 35-50 nm and the Ag layer has a thickness of about 10-15 nm. The InCeO films were deposited by sputtering a target that was formed by doping 10-30% CeO2 into In2O3. The cerium can effectively block the diffusion of oxygen atoms from the InCeO films to the InCeO/Ag interfacial layer. On the other hand, the Ag layer actually contains 1 atom percent Au and 0.5 atom percent Cu to enhance the stability of Ag atoms in the Ag layer. The design of the chemical compositions in both the InCeO and the Ag layers was reported to effectively improve the structural stability of the InCeO/Ag/InCeO films. The InCeO/Ag/InCeO films exhibit a low sheet resistance of 3-5 xcexa9/square and a high optical transmittance of above 90%. The deposition of InCeO/Ag/InCeO films films was also disclosed in U.S. Pat. No. 6,249,082 to Fukuyoshi et al., incorporated by this reference. However, the deposition of these films was only performed on a rigid glass substrate. The invention was not applied to the actual manufacture of information displays.
Other methods for producing such films are described in U.S. Pat. No. 4,166,876 to Chiba et al., U.S. Pat. No. 4,345,000 to Kawazoe et al., U.S. Pat. No. 4,451,525 to Kawazoe et al., U.S. Pat. No. 4,936,964 to Nakamura, U.S. Pat. No. 5,178,957 to Kolpe et al., U.S. Pat. No. 6,171,663 to Hanada et al., U.S. Published Patent Application No. US 2001/0050222 by Choi et al., PCT Patent Publication No. WO 98/12596 by Staral et al., European Patent Publication No. EP 1041644 by Cheung, and European Patent Publication No. EP 1155818, all of which are incorporated herein by this reference.
Accordingly, there is still a need for an improved method of preparing conductive films on a flexible substrate. There is a particular need for methods that can be used to prepare such films in a continuous, roll-to-roll manner, so that the films can be collected in continuous rolls.
In general; a method for forming a composite film according to the present invention comprises:
(a) providing a flexible plastic substrate;
(b) depositing a multi-layered conductive metallic film continuously on the flexible plastic substrate to form a composite film; and
(c) collecting the composite film in continuous rolls.
The thin-film deposition technique can be DC or RF magnetron sputtering, ion beam deposition, chemical vapor deposition, ion beam enhanced deposition, and laser ablation deposition. A preferred thin film deposition technique is DC magnetron sputtering.
Typically, the thin film that is deposited is an InCeO/Ag/InCeO film with three layers, with exterior InCeO layers surrounding an interior Ag layer. However, methods according to the present invention can be used to deposit other metallic films.
Typically, the DC magnetron sputtering is performed in an atmosphere that contains argon, and, optionally, oxygen. Preferably, the atmosphere contains oxygen in the deposition of InCeO films. The oxygen concentration can be varied to optimize the properties of the thin film being deposited.
The method can further comprise the step of purging the surface of the flexible plastic substrate with plasma prior to film deposition. The method can also further comprise a post-deposition annealing step.
Typically, the flexible plastic substrate is polyethylene terephthalate (PET), polyethylene naphthalate (PEN), polyethersulfone (PES), polycarbonate (PC), polysulfone, a phenolic resin, an epoxy resin, polyester, polyimide, polyetherester, polyetheramide, cellulose acetate, aliphatic polyurethanes, polyacrylonitrile, polytetrafluoroethylenes, polyvinylidene fluorides, poly(methyl xcex1-methacrylates) or an aliphatic or cyclic polyolefin. Aliphatic polyolefins include, but are not necessarily limited to, high density polyethylene (HDPE), low density polyethylene (LDPE), and polypropylene, including oriented polypropylene (OPP). Cyclic polyolefins include, but are not necessarily limited to, poly(bis(cyclopentadiene)). A preferred flexible plastic substrate is a cyclic polyolefin or a polyester. The flexible plastic substrate can be reinforced with a hard coating. Typically, the hard coating is an acrylic coating.
Typically, a composite film formed according to the present invention has superior properties, including high optical transmittance, low electrical resistance, high surface smoothness, high stability to exposure to high temperature and high humidity, high interlayer adhesion, and wet and dry etchability.
A further aspect of the invention is a novel composite film comprising a InCeO/Ag/InCeO metallic film coated or deposited on a flexible plastic substrate, wherein the composite film has a combination of properties including: transmittance of at least 80% throughout the visible region; an electrical resistance of no greater than about 20 xcexa9/square, preferably no greater than 10 xcexa9/square; a root-mean-square roughness of no greater than about 5 nm; and an interlayer adhesion between the InCeO/Ag/InCeO metallic film and the remainder of the composite film that is sufficiently great to survive a 180xc2x0 peel adhesion test. Preferably, the composite film further includes a reinforcing hard coating, preferably an acrylic coating, between the InCeO/Ag/InCeO metallic film and the flexible plastic substrate.
A preferred embodiment of the composite film includes the following properties: transmittance of at least 90% throughout the visible region; an electrical resistance of no greater than about 5 xcexa9/square; and a root-mean-square roughness of no greater than about 2.5 nm.
Typically, the flexible plastic substrate of the composite film is polyethylene terephthalate (PET), polyethylene naphthalate (PEN), polyethersulfone (PES), polycarbonate (PC), polysulfone, a phenolic resin, an epoxy resin, polyester, polyimide, polyetherester, polyetheramide, cellulose acetate, aliphatic polyurethanes, polyacrylonitrile, polytetrafluoroethylenes, polyvinylidene fluorides, poly(methyl xcex1-methacrylates) or an aliphatic or cyclic polyolefin, as described above. A preferred flexible plastic substrate is a cyclic polyolefin or a polyester.
Yet another aspect of the present invention is a multilayered electrode/substrate structure comprising an etched composite film made according to the present invention. The multilayered electrode/substrate structure can be an OLED or a PLED.
The conductive material can be a light-emitting polymer. The polymer can be poly(p-phenylenevinylene) (PPV), poly(dialkoxyphenylenevinylene), poly(thiophene), poly(fluorene), poly(phenylene), poly(phenylacetylene), poly(aniline), poly(3-alkylthiophene), poly(3-octylthiophene), or poly(N-vinylcarbazole).
Alternatively, the multilayered electrode/substrate structure can include a conductive material that is a luminescent organic or organometallic material. The luminescent organic or organometallic material can be selected from the group consisting of metal ion salts of 8-hydroxyquinolate, trivalent metal quinolate complexes, trivalent metal bridged quinolate complexes, Schiff base divalent metal complexes, tin (IV) metal complexes, metal acetylacetonate complexes, metal bidentate ligand complexes incorporating an organic ligand selected from the group consisting of 2-picolylketones, 2-quinaldylketones, and 2-(o-phenoxy) pyridine ketones, bisphosphonates, divalent metal maleonitriledithiolate complexes, molecular charge transfer complexes, rare earth mixed chelates, (5-hydroxy) quinoxaline metal complexes, and aluminum tris-quinolates.